With many hundreds of species of trees and shrubs found in any given region, and with some species available in multiple cultivars produced by selective breeding or grafting, it is no easy matter to identify a particular tree or shrub found in the wild or in a landscape. Yet it can be quite important to identify a specimen, such as when you need to diagnose and treat a problem.
The best method involves very careful observation of the plant's characteristics, combined with the use of one or more authoritative resource guides.
When you buy a plant at the store, it usually has an identification tag on it that tells you its name, species, and growing conditions. It is good practice to keep these plant tags for reference. If you did not retain the plant tag, the garden center can still be a good resource. If you know that your tree or shrub was purchased at a particular nursery, take a photo or a sample branch or leaf to the store and ask for the most knowledgeable staff member. Chances are good that he or she will be able to identify the plant for you.
There are many authoritative, scholarly reference books available that can help you identify a particular shrub or bush. These are often very expensive books, so it is best to consult them at a library at a university or arboretum. These reference books use a highly technical descriptive language, so you may need to learn a little bit about how plants are categorized and described. Most of these books will, however, offer a key to explain the terminology used and how to spot the different distinguishing features of trees and shrubs.
Most such books use a process called a dichotomous key, in which you systematically look for increasingly specific features, beginning with general shape and size of the overall plant, and proceed down to minute features, such as the number of lobes on a leaf or the texture of the bark. Gradually, you narrow the selection down until reaching the specific species, and perhaps even the specific cultivar within that species.
One excellent source of identification of trees and shrubs is Michael Dirr's "Manual of Woody Landscape Plants," regarded by many as the definitive printed source. Another good source is the Peterson Field Guide series. This series includes books on trees and shrubs for specific regions, complete with a dichotomous key to help sort them all out. The Peterson Guides are written for the non-expert reader.
A number of online versions of the dichotomous key approach are now available, in which a systematic definition of key features gradually narrows you down to precise identification of a tree or shrub. One excellent source is the "What Tree Is That?" website, sponsored by the Arbor Day Foundation. This tool leads you through a series of easy questions regarding leaf shape and other factors to bring you to accurate identification of various North American trees.
Most major universities maintain websites that catalog the various trees and shrubs growing in the region. By simple comparison with photos, you can often identify a particular tree or shrub. A call or email to a university extension service may be able to identify your tree in a matter of moments, especially if you have taken careful notes and have a photo of your specimen you can email.
Computer/Tablet/Smart Phone Apps
There are also several apps that can help you identify trees and shrubs, as well as other plants. Some even have photo recognition, where the app can identify the plant in a moment by comparing a photo you take with a huge database of information. One such app is the PlantSnapp app for iOS. Another excellent app is the Leafsnap app, developed for iOS in cooperation with the University of Maryland, Columbia University, and the Smithsonian Institution.
Careful Observation Is the Key
Whatever means you use to identify a tree or shrub, careful observation will make it easier. First, write down as much as you can about the plant. Take a picture to help you remember the characteristics. For flowering plants, cut off a stem with flowers, if possible.
The identification process can be accomplished either with a book, a website, or direct consultation with a knowledgeable person. In any case, you'll be answering questions that are presented. At each step, you may be asked to choose among two or more options for each question. This will lead you to another question. Once you have completed enough questions to narrow down the genus and species, you'll be left with a definitive identification of your tree or shrub.